Problem: $ { {5} \times \left[\begin{array}{r}{4} \\ {-2} \\ {-1}\end{array}\right] = {?} } $
Explanation: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{5}\times{4} \\ {5}\times{-2} \\ {5}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{20} \\ {-10} \\ {-5}\end{array}\right]}$